Phase behavior of CO2-shale oil in nanopores

Considering the interactions between fluid molecules and pore walls, variations in critical properties, capillary forces, and the influence of the adsorbed phase, this study investigates the phase behavior of the CO₂-shale oil within nanopores by utilizing a modified Peng-Robinson (PR) equation of state alongside a three-phase (gas-liquid-adsorbed) equilibrium calculation method. The results reveal that nano-confinement effects of the pores lead to a decrease in both critical temperature and critical pressure of fluids as pore size diminishes. Specifically, CO₂ acts to inhibit the reduction of the critical temperature of the system while promoting the decrease in critical pressure. Furthermore, an increase in the mole fraction of CO₂ causes the critical point of the system to shift leftward and reduces the area of the phase envelope. In the shale reservoirs of Block A in Gulong of the Daqing Oilfield, China, pronounced confinement effects are observed. At a pore diameter of 10 nm, reservoir fluids progressively exhibit characteristics typical of condensate gas reservoirs. Notably, the CO₂ content in liquid in 10 nm pores increases by 20.0% compared to that in 100 nm pores, while the CO₂ content in gas decreases by 10.8%. These findings indicate that confinement effects enhance CO₂ mass transfer within nanopores, thereby facilitating CO₂ sequestration and improving microscopic oil recovery.